FIG. 27 is a perspective view showing an exemplary photo-lithographically patterned spring structure 2700 (sometimes referred to as a “microspring”) that has been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. Spring structure 2700 includes a spring metal finger 2720 having a fixed end (anchor portion) 2720A secured to an underlying substrate 2701, a free end 2720B defining a tip 2720T, and a central section 2720C extending between fixed end 2720A and free end 2720B. Fixed end 2720A is connected to substrate 2701 by way of an intervening release material portion 2710 such that a cantilevered structure is formed with free end 2720B and central portion 2720C extending over an upper surface 2702 of substrate 2701. Spring metal finger 2720 is typically formed from a stress-engineered metal film having an internal stress gradient (i.e., a metal film fabricated such that its lower portions have a higher internal compressive stress than its upper portions) that is at least partially formed on a release material layer. This internal stress gradient causes central portion 2720C to bend away from substrate 2701 when the release material located under central section 2720C is etched away during a so-called “release” process. The internal stress gradient is produced in the spring metal by, for example, layering different metals having the desired stress characteristics, or using a single metal by altering the fabrication parameters. Such spring metal structures may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, scanning probes and actuated mirrors. For example, when utilized in a probe card application, the spring structure tip is brought into contact with a contact pad formed on an integrated circuit, and signals are passed between the integrated circuit and test equipment via the probe card (i.e., using the spring metal structure as a conductor). Other examples of such spring structures are disclosed in U.S. Pat. No. 3,842,189 (Southgate) and U.S. Pat. No. 5,613,861 (Smith).
As indicated in FIG. 27, a critical feature of spring structure 2700 is its tip height H, which is typically measured from tip 2720T to upper surface 2702 of substrate 2701 in a direction perpendicular to upper surface 2702. Tip height H is a product of the length of spring metal finger 2720 and the amount of bending produced in central section 2720C by the internal stress gradient upon release. As mentioned above, the length of spring metal finger 2720 is defined using well-established photo-lithographic techniques, and the stress gradient is defined, for example, by manipulating the process parameters during formation of the stress-engineered film. Therefore, to produce spring structures according to conventional manufacturing methods that meet a customer's specification (i.e., having a specified “target” tip height and specific spring constant), a spring structure manufacturer will select a spring film material, spring finger length, and set of process parameters that will produce the desired spring structure.
According to conventional methods currently utilized by spring structure manufacturers, stress calibration runs are used to determine the optimal process parameters needed to produce spring structures that meet a customer's specifications. For example, when a stress-engineered film is formed by sputter deposition, a calibration run is used to determine the rate at which chamber pressure is changed in order to produce the desired stress gradient (and, thus, the target tip height). It typically takes several calibration runs to identify the optimal process parameters needed to produce spring structures having the target tip height upon release. The thus-determined optimal process parameters are then repeatedly used to produce several batches (production runs) of spring structures meeting the customer's specifications. That is, the conventional method assumes that these determined optimal process parameters to be valid for many production runs following a successful stress calibration run.
A problem with the conventional method of relying on stress calibration runs to determine optimal process parameters is that, in practice, the operating parameters of fabrication (e.g., deposition chamber) equipment tend to drift over the course of several production runs, for example, due to target wear. For example, as indicated in FIG. 28(A), optimal process parameters may be determined to produce a desired spring structure 2700A, but parameter drift may cause an undesirable internal stress gradient, resulting in an actual spring structure 2700B having a release height HB that is less than the target release (tip) height HT. Conversely, as indicated in FIG. 28(B), the process parameters may drift such that an actual spring structure 2700C is produced having a release height HC that is greater than the target release height HT. This process parameter drift may be attributed to various factors, such as target erosion and chamber history, which are typically not easily detectable during the sequence of production runs. Accordingly, the stress gradients of the stress-engineered films tend to change over the course of several production runs. Further, because the deposition of the stress-engineered film is typically performed at a different time and in different equipment from the release process, it is difficult for a manufacturer to detect when the process parameters have drifted to a point where the resulting spring structures are no longer usable. Because several hundred spring structures are typically produced during each production run, a significant number of unacceptable (“out-of-spec”) spring structures are typically discarded to produce a suitable number of acceptable (“in-spec”) spring structures, which results in low production yields and high overall production costs.
One conventional approach used to avoid the parameter drift problem described above is to frequently recalibrate the deposition equipment. That is, after a successful calibration run, the thus-identified optimal process parameters are used for a number of runs, and then one or more additional calibration runs are used to identify drift and recalibrate the process parameters. However, frequently recalibration is a time-consuming procedure that greatly increases production costs. Thus, the parameter drift problem described above presents a serious obstacle for the industrial (mass fabrication) implementation of photo-lithographically produced spring structures.
Accordingly, what is needed is a method for fabricating photo-lithographically produced spring structures that does not rely on process (e.g., sputter) parameters to define a specific release height.